This same notion leads to the origin of evil: it is the absence of good. See these articles by Saint Thomas Aquinas. It is for this reason that evil is self-limiting, and even eventually destroys itself, for the privation of good leads ultimately to non-being. You can increase the size of a doughnut hole until the doughnut itself, and likewise its hole, completely disappears.
Enlightenment religion (a term I use for want of any better, actual practitioners would likely reject it) is based on doubt and is essentially skeptical. It attempts to minimize or eliminate truth-claims, especially those claims of a metaphysical or spiritual character. It concentrates on the hole rather on the doughnut surrounding it. For this reason, Enlightenment religion has to pierce an existing religion — any religion will do — as a hole in a doughnut is like a hole in a bagel, and cannot have an independent existence.
You can take so much away from a religion until all that is left is the name only.
Senator Obama apparently subscribes to this kind of skeptical religion. See the article: Obama's Religious Ruse: 'I've Always Been a Christian'.
It is hazardous sometimes to argue philosophy with a skeptic. Confronting them with the slippery slope argument may actually convince them that sliding down that slope would be a thrilling ride. Instead, we must perhaps encourage them to hold onto whatever faith they do have, no matter how small, and help it grow.